X-ray and gamma ray sources are presently being used in a wide array of medical and industrial machinery, and the breadth of such use expands from year to year. Consumers tend to notice medical and dental X-ray machines, but in addition to these applications there are baggage screening machines, CAT scan machines, non-destructive industrial inspection machinery and ion implantation machines used in the manufacture of silicon wafer computer chips. All require that radiation be contained and directed. In particular, the ion implantation machinery increased in the 1980s and 1990's with the silicon chip boom.
In the past, lead itself or lead-polymer composites were used to make such items. But there are numerous problems with the use of lead. One problem with lead is that it is toxic and thus subject to increasingly stringent legal controls. Another issue is that lead may not have the mechanical or electrical properties desired for a given application. Lead has been used in various forms in wide range of applications: machined, as a solid casting, as a solid encased within a matrix such as a polymer matrix, or as a filler. As a filler, it may be lead particles, tribasic lead-sulfate or lead-oxide particles or particles of a specified shape or size, or as a mixture with other materials such as tin. Tungsten shielding, or polymer-tungsten shielding has also been used. Examples of all of these methods may be found in the prior art.
Polymer-metal composite materials are of increasing importance in radiation technology and a number of industries, due to the fact that polymer-metal composite materials offer characteristics which are difficult or impossible to match in other materials of equivalent price or ease of manufacture.
In general, polymer-metal composites are materials having a polymer matrix containing particles of a metal compound intermixed therein. The polymer may advantageously have plastic properties allowing for ease of manufacture, but a wide variety of polymers are known for use in such composites. The choice of metal will place undesirable limitations on the range of properties which may be provided to the manufactured composite. In general, high density and accompanying factors such as increased mass, increased radiological shielding properties, heat-deflection properties, impact strength, tensile strength and so on. In the prior art, lead has been a particularly favored material for its density and ease of working. Tungsten has been favored more recently, despite cost concerns. Three characteristics in particular which make such materials desirable are electrical non-conductivity, radiological shielding ability, and high density.
There is a growing list of applications for which polymer-metal composite materials are either required or advantageous. Reactor shielding, ion implantation machine source insulators, X-ray tube housings, radioisotope housings, syringe housings, body shielding, dental X-ray packets (“bitewings”), containers, other castings and housings all benefit from the properties of polymer-metal composite materials. In the case of typical high voltage insulators for ion implantation machinery, a thick walled generally round or cylindrical part is created out of lead or polymer-lead-oxide ranging from an inch to several feet or more in long dimension and weighing anywhere up to 500 pounds. Wall thickness may range from ½ inch to several inches. Such parts must resist high voltages, shield against x-ray or gamma ray emission and hold a high vacuum state when connected to the vacuum chamber. X-ray tube shielding is generally thinner (often 0.070 inch thickness), generally smaller, and of different shape, having an aperture for the X-ray beam, but once again must offer high voltage insulation and radiation protection. The lead in such devices obviously presents an environmental challenge to manufacture, use and disposal.
In the processing of lead precursor filled plastics known in the art, specialized facilities, handling procedures, training and safety equipment must be used to protect the employees from the lead precursor they handle. Lead-based dust is a particular concern, being airborne and inhalable. Such dust may be generated during mixing, molding, deflashing, machining and finishing of final products such as insulators or shields, to say nothing of earlier stages of mining, smelting and refining of lead and the final disposal of the used product at the end of its useful life. Even during the life span of the product, it is illegal to sand, machine, alter or use the product in any way that will generate dust. All such processes must be carried out at special lead handling sites, and all waste dust from any of these processes must be collected in accordance with OSHA regulations and transported to hazardous waste land fills in accordance with OSHA and DES guidelines.
Internalized by law into the manufacturing process, such safety issues dramatically increase the cost of such products, which in turn increases other medical or industrial costs.
One recent invention to deal with this issue is TUNGSTEN-PRECURSOR COMPOSITE, for which application Ser. No. 10/095,350 was filed Mar. 9, 2002, in the name of the same inventor, Stuart J. McCord, and issued on May 6, 2004, as U.S. Pat. No. 6,740,260. This invention addresses material and cost concerns of tungsten shielding by proposing the use of tungsten precursor materials which testing reveals to have favorable properties. However, an entire range of desirable properties is not attainable with a single family of compounds, and so additional compounds may be desirable in order to expand the range of properties which may be attained in a lead-free shield device. Cost, of course, is one issue. Availability is another, as are actual material properties. During prosecution of that patent, U.S. Pat. No. 5,548,125 issued to Sandback (RADIATION PROTECTIVE GLOVE) and U.S. Pat. No. 4,957,943 issued to McAllister et al (PARTICLE-FILLED MICROPOROUS MATERIALS) were cited by the examiner prior to allowance. Another attempt to deal with the issue of environmental lead contamination may be found in U.S. Pat. No. 6,048,379 issued Apr. 11, 2000, to Bray et al for “HIGH DENSITY COMPOSITE MATERIAL.” This patent teaches the use of tungsten powder, a binder and a polymer to provide a composite material offering a density high enough for use as ammunition. As stated, a serious issue with the use of tungsten is that of cost. Tungsten metal is quite expensive in comparison to lead. For example, tungsten-composite materials may cost as much as $20 per pound.
U.S. Pat. No. 5,730,664, U.S. Pat. No. 5,719,352, and U.S. Pat. No. 5,665,808, respectively issued to Asakura, Griffin, Bilsbury all disclose metal-polymer composites for projectiles, respectively golf balls and shot pellets. Other patents from the same art (projectiles) also propose non-toxic materials.
In the actual radiation shielding art itself, various patents propose polymer-metal composites of various forms.
EcoMASS (a registered trademark of the PolyOne Corporation) is a combination of tungsten metal and nylon and elastomer compounds used for shielding, apparently based upon the Bray '379 patent related to ammunition and thus developed specifically in response to military/sporting needs for non-toxic ammunition. It does not teach that materials other than tungsten may be used, thus limiting the range of characteristics of the final product. For example, tungsten is electrically conductive and thus is not normally suitable for insulators. As mentioned earlier, this material also faces cost limitations. In addition, this material has manufacturing limitations in terms of thickness and size of the final item.
U.S. Pat. No. 4,619,963 issued Oct. 28, 1986, to Shoji et al for “RADIATION SHIELDING COMPOSITE SHEET MATERIAL” teaches a lead-tin fiber and resin shield, as does U.S. Pat. No. 4,485,838 issued Dec. 4, 1984, to the same inventors.
U.S. Pat. No. 6,310,355 issued Oct. 30, 2001, to Cadwalader for “LIGHTWEIGHT RADIATION SHIELD SYSTEM” teaches a flexible matrix having a radiation attenuating material and at least one void.
U.S. Pat. No. 6,166,390 issued Dec. 26, 2000, to Quapp et al for “RADIATION 10 SHIELDING COMPOSITION” teaches a concrete composite material.
U.S. Pat. No. 5,360,666 issued Nov. 1, 1994, and U.S. Pat. No. 5,190,990 issued Mar. 2, 1993, to Eichmiller for “DEVICE AND METHOD FOR SHIELDING HEALTHY TISSUE DURING RADIATION THERAPY” teach a radiation shield for the human body comprising an elastomeric material and certain mixtures (see the summary of the invention) of various metals in the form of spherical particles.
Bismuth is one of the least electrically and thermally conductive metals, and in non-radiological applications, it has been known as a substitute for lead. For example, in glazes and surface treatments bismuth is known to provide high gloss, similar surface material properties, viscosity and resistance to detergents (dishwasher detergents). In optical work, bismuth oxide is known to be useful in replacing lead (in amounts of 50% or less) so as to increase specific gravity, refractive index and durability of the optical equipment.
The safety of bismuth oxide may be understood from the fact that it is commonly used in internal prosthesis (bone replacement), in order to create a “radiologically opaque” or “X-ray opaque” part. Such items show up on an X-ray at the low power of radiation which a patient receives, thus marking the location and structure of the prosthetic appliance which has been physically implanted in the body of the recipient.
However, bismuth is not generally known in the radiological field as an X-ray resistant shielding material.